Method of preparing zirconia-silica xerogels, the xerogels, and xerogel catalysts

ABSTRACT

A method of preparing large pore volume zirconia-silica catalyst supports, the resulting supports and a method of polymerizing olefins comprising contacting an olefin or mixture of olefins with a catalyst prepared with these supports. The supports are prepared by reacting a zirconium compound of the formula M 4  Zr(C 2  O 4 ) 4 .nH 2  O, where M is an alkali metal or ammonium ion and n equals 0 to 10, with a silicon compound of the type A 2  SiO 3 , where A is an alkali metal, in an aqueous solution at a pH equal to at least 11, then adding an acidic solution to a pH of about 5-9 to produce a hydrocogel. The hydrocogel is then aged and washed free of soluble by-products first with water, then with aqueous ammonium nitrate and again with water. The water is removed from the washed hydrocogel by azeotropic distillation or by washing with a water miscible solvent and then calcining the resulting xerocogel.

BACKGROUND OF THE INVENTION

One of the features of this invention is to prepare an improved largepore volume zirconia-silica catalyst support by reacting a particularzirconium compound with a particular silicate to produce a hydrocogel,aging the hydrocogel, washing with water and then aqueous ammoniumnitrate and again with water, removing water from the resulting washedhydrocogel to produce a xerocogel either by azeotropic distillation orby leaching with a water miscible solvent and calcining the resultingxerocogel.

Another feature of the invention is to provide a catalyst supportprepared by the above method.

A further feature of the invention is a method of polymerizing olefinscomprising contacting the olefins with a catalyst comprising the abovesupport and a chromium compound associated with it under polymerizingconditions.

The most pertinent prior art of which we are aware are the following:

U.S. Pat. No. 2,289,919 discloses purified silica hydrogel (free ofsodium ion) suspended in a zirconium salt solution (aqueous zirconylchloride). Aqueous ammonium hydroxide is then added in order toprecipitate zirconia onto the hydrogel (not coprecipitated). Thezirconia-silica mixture is washed, dried at 300° F., and calcined.

U.S. Pat. No. 2,444,913 discloses a method of preparing plural oxidecatalysts containing zirconia and silica which comprises preparing asolution containing silica and zirconia by mixing an alkali metalsilicate solution with an alkali metal zirconium carbonate solution andcoprecipitating the silica and zirconia in said solution by the additionof an acid to a pH of about 6.5. The hydrogel is dried at 200°-210° F.and calcined at 1400° F. in an air-steam.

U.S. Pat. No. 3,950,316 discloses a method of preparing a silica-titaniacatalyst support comprising mixing a water soluble titanium compound(potassium titanium oxalate, K₂ TiO(C₂ O₄)₂.2H₂ O or ammonium titaniumoxalate, (NH₄)₂ TiO(C₂ O₄)₂.H₂ O with an alkali metal silicate solution,said titanium compound being nonreactive with the silicate, adding anacidic material to the silicate cntaining the titanium compound to forma hydrogel, aging the hydrogel for more than one hour, washing the agedhydrogel with either an ammonium salt solution or a dilute acid toproduce an alkali-free hydrogel, forming a mixture comprising saidwashed hydrogel and a normally liquid oxygen-containing water solubleorganic azeotrope-forming compound, separating the organic compound withwater to form a xerogel. In the present invention, in contrast, thezirconium oxalate is reactive with the silicate. For example, ammoniagas is liberated when ammonium zirconium oxalate is mixed with sodiumsilicate solution.

U.S. Pat. No. 3,862,104 is similar to the above U.S. Pat. No. 3,950,316except potassium titanate oxalate in aqueous solution with sodiumsilicate is added to an ammonium sulfate solution.

U.S. Pat. No. 3,801,705 discloses a method for providing a silicaxerogel having a narrow pore diameter distribution within the range300-600Å, a surface area within the range 200-500 m² /g, and a largepore volume between 2-3.5 cc/g.

None of the above discloses the claimed invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plot of the nitrogen pore volume verses the calcinationtemperature for Xerogels D and E.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this invention an alkali metal or ammonium zirconium oxalate compoundis used as the zirconia source during coprecipitation with silica froman alkali metal silicate in the presence of an acidic compound. Thesenew catalyst supports have increased thermal stability towards highcatalyst activation temperatures and produce resins with increased meltindex when used as a polymerization catalyst support, especially withcatalysts based on chromium (III) acetylacetonate particularly with thistype of catalyst disclosed in prior U.S. Pat. No. 3,953,413, assigned tothe assignee hereof.

The preparation of large pore volume zirconia-silica is bycoprecipitating or copolymerizing hydrous zirconia and hydrous silica bydissolving or reacting a zirconium compound of the type M₄ Zr(C₂O₄)₄.nH₂ O, where M is an alkali metal or ammonium ion and n equals 0 to10, with a silicon compound of the type A₂ SiO₃, where A is an alkalimetal, in aqueous solution at a pH of at least 11.0 followed by theaddition of an acidic compound such as sulfuric acid, hydrochloric acid,or ammonium sulfate to a pH of about 5-9. The resulting hydrocogel isthen aged at ambient to 90° C. for at least one hour followed by washingwith water, aqueous ammonium nitrate, and again water. Water removalfrom the washed hydrocogel to produce the xerocogel is accomplished byazeotropic distillation with compounds capable of forming an azeotropewith water, for example, ethyl acetate or benzene, or by washing thehydrocogel with a water miscible solvent such as acetone. The finalxerocogel is calcined at a temperature in the range of about 1000°-1800°F. prior to use as an olefin polymerization catalyst support. Axerocogel having a nitrogen pore volume in the range of about 1.5 to 3.5cc/g, a surface area in the range of about 200-600 m² /g. and a porediameter in the range of about 200 Å to 600 Å is obtained. Theconcentration of zirconia (ZrO₂) in the xerocogel is in the range ofabout 0.1 to 67.5 weight percent, preferably about 1 to 5 weightpercent.

It is desirable that both the zirconium compound and the alkali metalsilicate be together in solution prior to cogel precipitation by strongacid. The pH of this solution determines whether premature precipitationof zirconia takes place completely or in part. If prematureprecipitation of zirconia takes place, it will obviously not becoprecipitated with silica and will threfore be lost by a necessaryfiltration step prior to coprecipitation by strong acid. Therefore, thepH of a solution of the zirconium compound and the alkali metal silicateshould not drop below about 11.0 prior to the coprecipitation ofzirconia and silica by strong acid.

The presence of oxalate anions insures or increases the solubility ofthe zirconium compound at a particular pH. For example, when a solutionof zirconium sulfate (pH=2.0) was used in place of ammoniumtetraoxalatozirconate and was added to a solution of sodium silicate(pH=11.6), substantial premature precipitation of hydrous zirconia tookplace even at a pH as high as 11.4. This necessitated removal byfiltration, and therefore loss of zirconia, prior to strong acidpromoted coprecipitation of what zirconium was left in solution. Thus,the targeted 2 wt. % zirconium in the final xerocogel was only 1.6 wt.%.

The preparation of the coprecipitated zirconia-silica and silicacatalyst supports of this invention is illustrated by the followingExamples A-E.

EXAMPLE A Lithium Tetraoxalatozirconate (IV) Hydrate as the ZirconiaSource

A source of 15 grams of lithium tetraoxalatozirconate (IV) hydrate in400 cc deionized water (pH=3.4) was added to a stirred solution of 400grams of sodium silicate (% Na₂ O, 6.75; % SiO₂, 25.3; wt. ratio SiO₂/Na₂ O, 3.75) in 800 cc. deionized water. The cloudy solution was thenfiltered. To the filtered solution, 12.75% sulfuric acid was addeddropwise with stirring to a pH of 6. The coprecipitated hydrocogel wasaged at about 90° C. for at least one hour during which time the pH wascontrolled at 6. The aged coprecipitated hydrocogel was then suctionfiltered and washed first with deionized water, a solution of 1% aqueousammonium nitrate, and finally deionized water until sodium, lithium andsulfate ions were as completely removed as possible. Water was removedazeotropically with ethyl acetate. The recovered xerocogel was calcinedat 1500° F. in air prior to use as a catalyst support (see Example 5 ofthe Table). Weight percent zirconium was 1.87, as zirconia 2.6 andnitrogen pore volume was 2.29 cc/g.

EXAMPLE B Sodium Tetraoxalatozirconate (IV) Hydrate as the ZirconiaSource

A solution of 66 grams of sodium tetraoxalatozirconate (IV) hydrate in2400 cc deionized water (ph=4.5) was added to a stirred solution of 2400grams sodium silicate (% Na₂ O, 6.75; % SiO₂, 25.3; wt. ratio SiO₂ /Na₂O, 3.75) in 4800 cc deionized water. Since an essentially clear solutionwas obtained, no filtration step was required. To the solution,initially at a pH of 11, was added 1800 cc of a 12.75% aqueous sulfuricacid solution dropwise with stirring. A pH of 6 as reached. Theprecipitated hydrocogel was then aged at about 90° C. for at least onehour, for example 10 hours. An additional 25 cc of 12.75% sulfuric acidwas required to maintain the pH at 6. Following aging the hydrocogel wassuction filtered and washed first with deionized water, a solution of 1%aqueous ammonium nitrate, and finally deionized water until sodium andsulfate ions were recovered. The bulk of the water was removed from thehydrocogel by acetone; the remaining water was removed azeotropicallywith a mixture of n-heptane and n-hexane. The recovered xerocogel wascalcined at 1500° F. in air prior to use as a catalyst support (seeExample 6 of the Table). Weight percent zirconium was 1.3, as zirconia1.8 and nitrogen pore volume was 1.35 cc/g.

EXAMPLE C Potassium Tetraoxalatozirconate (IV) Hydrate as the ZirconiaSource

A solution of 111 grams potassium tetraoxalatozirconate (IV) hydrate in2400 cc deionized water (pH=6.7) was added to a stirred solution of 2400grams sodium silicate (% Na₂ O, 6.75; % SiO₂, 25.3; wt. ratio SiO₂ /Na₂O, 3.75) in 4800 cc deionized water. The resulting cloudy solution wasfiltered to remove small amounts of suspended solid. To the filteredsolution, initially at a pH of 10.7 was added a total of 1680 cc of12.75% sulfuric acid dropwise with stirring. Aging at a pH of about 6.0was for at least one hour at 90° C., for example, 10 hours. The final pHwas 6.3. Following aging the hydrocogel was suction filtered, washedwith deionized water, 1% ammonium nitrate, and again deionized water.Water was removed from the hydrocogel azeotropically with ethyl acetate.The recovered xerocogel was calcined at 1500° F. in air prior to beingused as a catalyst support (see Example 7 of the Table). Weight percentzirconium was 2.1 and a percent zirconia 2.8.

EXAMPLE D Ammonium Tetraoxalatozirconate (IV) Pentahydrate as theZirconia Source

A solution of 13 grams ammonium tetraoxalatozirconate (IV) pentahydratein 400 cc deionized water (pH=3.5) was added at about 20° C. to astirred solution of 400 grams sodium silicate (% Na₂ O, 6.75; % SiO₂,25.3; wt. ratio SiO₂ /Na₂ O, 3.75) in 800 cc deionized water. A slightlycloudy solution with a pH of 11.3 was obtained. Dilute sulfuric acid(12.75%) was then added until a pH of about 6 as reached. The resultingcoprecipitate was then aged for at least one hour, for example 4 hours,at about 90° C. while maintaining the pH at 6.0 with additional dilutesulfuric acid. Following aging, the hydrocogel was collected byfiltration and washed first with deionized water, a solution of 1%ammonium nitrate, and finally with deionized water. Water was removedazeotropically with ethyl acetate. The recovered xerocogel was air driedup to 80° C. to remove absorbed ethyl acetate. The xercogel wasseparated into four 1 to 2 gram portions which were calcined for onehour at 300° F., 1500° F., 1700° F. and 1750° F., respectively. Eachsample was then analyzed for nitrogen pore volume. Results are shown inFIG. 1. Comparison of pore volume data with that of the xerogel ofExample E clearly shows that the tendency for pores to collapse withheating is reduced by the presence of zirconia with silica as acoprecipitate.

A larger portion of the recovered xerogel was calcined at 1500° F. inair prior to use as a catalyst support (see Example 8 of the Table). Thenitrogen pore volume was 2.45 cc/gram.

EXAMPLE E No Zirconia

Dilute aqueous sulfuric acid (12.6) was added to a stirred solution of200 grams sodium silicate (% Na₂ O, 6.75; % SiO₂, 25.3; wt. ratio SiO₂/Na₂ O, 3.75) in 500 cc deionized water (pH=11.6). When a pH of 6 wasreached, the slurry was aged by heating at least one hour, for example,2 hours, with stirring at about 90° C. while maintaining the pH at 6 byadditional dilute sulfuric acid. After aging, the hydrogel was recoveredby filtration and washed first with deionized water, a solution of 1%ammonium nitrate, and finally with deionized water in order to removeall soluble by-products. Water was removed azeotropically with ethylacetate. The recovered xerogel was then air dried to remove absorbedethyl acetate.

One to two gram samples of the xerogel were calcined for one hour at300° F., 1500° F., 1700° F. and 1750° F., respectively. Each sample wasthen subjected to a nitrogen pore volume determination. Results arepresented in FIG. 1. One can easily see the fall-off in pore volume withincreased calcination temperature.

A larger portion of the recovered xerogel was calcined at 1500° F. inair prior to use as a catalyst support (see Example 4 of the Table). Thenitrogen pore volume was 1.84 cc/gram.

Catalyst Preparation

The preparation of olefin polymerization catalysts having as a supportthe coprecipitated zirconia-silica is illustrated as follows:

Chromium (III) acetylacetonate type catalysts were prepared by drymixing the chromium chelate with the calcined xerocogel (Examples 5, 6,7 and 8 of of the Table). Heat activations were in a non-oxidizingatmosphere at 800°-2000° F., such as at 1700° F. in nitrogen, followedby a 30 minute dry air treatment at 1300° C. in a fluid bed. Chromiumchelate catalysts based on a moderate pore volume silica (Example 2) ona moderate pore volume silica containing surface absorbed zirconia(Example 3) or on an initially high pore volume silica (Example 4) wereprepared and activated in a similar fashion.

All catalysts contained 1 wt. % chromium.

The polymerization of olefins using the catalyst of this invention isillustrated by the polymerization of ethylene. A specific example ofthis is as follows:

Resin synthesis was in a one gallon autoclave with isobutane as diluentand under 550 psi pressure. Ethylene was fed on demand. synthesisconditions were at 225° F. (see Table).

The preparation of the catalysts using these supports and polymerizationof ethylene to polyethylene are illustrated by the accompanying Tableand can be summarized as follows:

EXAMPLE 1

A commercial Phillips type chromium (VI) oxide catalyst based on amoderate pore volume (1.65 cc/gram), zirconia-free silica and activatedin air at 1700° F. produced a polyethylene resin a 225° F. with a lowmilled melt index of 0.1.

EXAMPLE 2

A chromium (III) acetylacetonate type catalyst based on the samemoderate pore volume, zirconia-free support as described in Example 1,activated at 1700° F. in nitrogen followed by air treatment at 1300° F.produced a polyethylene resin at 225° F. with an improved milled meltindex of 0.4, still a relatively low value.

EXAMPLE 3

This example shows that a chromium (III) acetylacetonate catalyst basedon the same moderate pore volume support as in Examples 1 and 2, butcontaining surface zirconia, showed no improvement in resin melt index.

EXAMPLE 4

The chromium (III) acetylacetonate type catalyst of this example wasbased on a high pore volume zirconia-free silica and demonstrates thatalthough the pore volume was initially high, 2.30 cc/gram, the resinmelt index was only 2.3 due to the collapse of unstabilized pores. Underthe 1700° F. activation conditions of the catalyst, the pore volume wasreduced to 1.64 cc/gram.

EXAMPLE 5

A chromium (III) acetylacetonate catalyst based on a high pore volumesilica containing coprecipitated zirconia from lithiumtetraoxalatozirconate (IV) hydrate, activated at 1700° F. followed byair treatment at 1300° F., produced a resin at 225° F. with a melt indexalmost double, 4.1, that from a resin derived from a zirconia-freecatalyst (Example 4).

EXAMPLE 6

This chromium (III) acetylacetonate type catalyst was based on amoderately high pore volume silica containing coprecipitated zirconiafrom sodium tetraoxalatozirconate (IV) hydrate. This catalyst may becompared to that of Example 2, also a moderate pore volume catalyst. Theresin melt index was 0.9, again higher than the melt index of a resinproduced by a nearly equivalent catalyst without zirconia.

EXAMPLE 7

This chromium (III) acetylacetonate type catalyst was based on amoderate high pore volume silica containing coprecipitated zirconia frompotassium tetraoxalatozironate (IV) hydrate. A review of the data againshows improved resin melt index.

EXAMPLE 8

A chromium (III) acetylacetonate type catalyst was based on a high porevolume silica containing coprecipitated zirconia from ammoniumtetraoxalatozirconate (IV) hydrate. The resin melt index of 3.4 was inclose agreement with the melt index of a resin produced from a similarcatalyst based on silica containing coprecipitated zirconia from lithiumtetraoxalatozircronate (IV) hydrate (Example 5).

The melt index of the polyethyelene prepared with the catalysts of thisinvention is dependent upon the source of zirconia. This is a surprisingresult. For example, the melt indices increase in the following orderwhere the zirconia source is as indicated:

    Na.sub.4 Zr(C.sub.2 O.sub.4).sub.4 <K.sub.4 Zr(C.sub.2 O.sub.4).sub.4 <(NH.sub.4).sub.4 Zr(C.sub.2 O.sub.4).sub.4 ≈Li.sub.4 Zr(C.sub.2 O.sub.4).sub.4

                                      TABLE    __________________________________________________________________________                    N.sub.2.sup.b                             Activation,                                   Resin.sup.f    Example         ZrO.sub.2                Wt. %                    P.V.,                       Chromium                             °F.                                   Synthesis                                         Milled    No.  Source.sup.a                ZrO.sub.2                    cc/g                       Source                             N.sub.2                                Air                                   Temp., °F.                                         MI.sup.c    __________________________________________________________________________    1    none   none                    1.65                       CrO.sub.3                             -- 1700                                   225   0.1    2    none   none                    1.65                       Cr(AcAc).sub.3.sup.e                             1700                                1300                                   225   0.4    3    ZrO(NO.sub.3).sub.2.sup.d                1.3 1.65                       Cr(AcAc).sub.3                             1700                                1300                                   225   0.4    4    none   none                    1.84                       Cr(AcAc).sub.3                             1700                                1300                                   225   2.3    5    LiZrO.sub.x                2.6 2.29                       Cr(AcAc).sub.3                             1700                                1300                                   225   4.1    6    NaZrO.sub.x                1.8 1.35                       Cr(AcAc).sub.3                             1700                                1300                                   225   0.9    7    KZrO.sub.x                2.8 -- Cr(AcAc).sub.3                             1700                                1300                                   225   1.3    8    NH.sub.4 ZrO.sub.x                2.7 2.45                       Cr(AcAc).sub.3                             1700                                1300                                   225   3.4    __________________________________________________________________________     Footnotes:     .sup.a Alkali metal or ammonium tetraoxalatozirconate(IV) hydrates, or     zirconyl bitrate as indicated     .sup.b Pore volume of xerocogel after calcination at 1500° F.     Catalysis, vol. II. pp. 111-1165, P.H. Rheinhold Publishing Corp. New     York, N.Y., 1955.     .sup. c Milled resin melt index, grams per 10 minutes; ASTM D1238-62T.     .sup.d Not coprecipitated but impregnated onto an existing silica xerogel     followed by calcination to provide zirconia of silica.     .sup.e chromium(III) acetylacetone.     .sup.f Resin synthesis conditions were at the temperature indicated, in     isobutane and under 550 psig total pressure.

All parts and percentages herein are by weight.

Having described our invention as related to the embodiments set outherein, it is our intention that the invention be not limited by any ofthe details of description, unless otherwise specified, but rather beconstrued broadly within its spirit and scope as set out in the appendedclaims.

We claim:
 1. The method of preparing large pore volume zirconia-silicauseful as a catalyst support for a catalyst for polymerizing andcopolymerizing olefins, comprising:a. reacting a zirconium compound ofthe formula M₄ Zr(C₂ O₄)₄.nH₂ O, where M is an alkali metal or ammoniumion and n equals 0 to 10, with a silicon compound of the type A₂ SiO₃,where A is an alkali metal, in a aqueous solution at a pH of at least 11and then adding an acidic material to a pH of about 5-9, to produce ahydrocogel; b. aging said hydrocogel at a temperature between aboutambient to 90° C. for at least one hour; c. washing said hydrocogel of b.[.first with water, then with aqueous ammonium nitrate and again withwater.]. .Iadd.free of soluble by-products with an aqueousliquid.Iaddend.; d. removing water from the resulting washed hydrocogelof c to produce a xerocogel by azeotropic distillation by mixing with acompound capable of forming an azeotrope with water or by washing thehydrocogel with a water miscible solvent to produce a substantiallywater-free, large pore volume zirconia-silica catalyst support; and e.calcining the resulting xerocogel at a temperature of about 1000°-1800°F. preparatory to its use as an olefin polymerization catalyst support.2. .[.the.]. .Iadd.The .Iaddend.method of claim 1 wherein M is an alkalimetal.
 3. The method of claim 1 wherein M is an ammonium ion.
 4. Themethod of claim 1 wherein said acidic compound in said aqueous solutionis sulfuric acid, hydrochloric acid or ammonium sulfate.
 5. The methodof claim 1 wherein water is removed from the washed hydrocogel byazeotropic distillation with a compound capable of forming an azeotropewith water.
 6. The method of claim 1 wherein water is removed by washingthe hydrocogel with a water miscible solvent.
 7. A catalyst supportprepared by the method of claim
 1. 8. A catalyst support prepared by themethod of claim
 2. 9. A catalyst support prepared by the method of claim3.
 10. A catalyst support prepared by the method of claim
 4. 11. Acatalyst support prepared by the method of claim
 5. 12. A catalystsupport prepared by the method of claim 6.